A positive electrode active material is used for the positive electrode of batteries. The batteries in general can be classified into two groups. These are a “primary battery” in which the electrochemical reaction is substantially irreversible and which cannot be used any more when once discharged, and a “secondary battery” in which the electrochemical reaction is reversible to enable “recharging” and which can be used a plurality of times. The characteristic feature of the secondary battery includes an environmental advantage thanks to decrease in the required replacement (i.e. decrease in waste), and this battery is used for various power sources.
Among secondary batteries, a lithium secondary battery has already been put into commercial use as a small power source for cellular phones, notebook computers and the like. Furthermore, a demand for a secondary battery as a large power source for electric vehicles, distributed power storages and the like is on the rise.
In the lithium secondary battery, a use of an olivine-type lithium iron phosphate (LiFePO4) as a positive electrode active material has been proposed. For example, in Examples of Patent Document 1, an olivine-type lithium iron phosphate is obtained from ammonium dihydrogenphosphate (NH4H2PO4), iron oxalate dihydrate (FeC2O4.2H2O) and lithium carbonate.
In order to eliminate the use of lithium used in the lithium secondary battery, it is considered to obtain a sodium secondary battery by using sodium which is one digit cheaper than lithium, abundant as a resource, and lightweight, and has a relative high standard potential. A use of a sodium secondary battery in place of the existing lithium secondary battery enables mass-production of a large secondary battery for electric vehicles, distributed power storages and the like, without fear of depletion of resources. More specific examples of the sodium secondary battery include a secondary battery using a sodium-containing positive electrode active material for the positive electrode and using metal sodium or sodium alloy for the negative electrode, and a secondary battery using a sodium-containing positive electrode active material for the positive electrode and using a carbonaceous material or the like for the negative electrode.
As regards a positive electrode active material used for the positive electrode of the sodium secondary battery, a positive electrode active material mainly composed of an olivine-type phosphate represented by the formula NaaMbPO4 (M is Mn, Fe, Co or the like) has been proposed. Synthesis of an olivine-type sodium iron phosphate by the following scheme is described, for example, in Patent Document 2.
(1) Thermal Carbonization Solid-Phase Synthesis0.5Na2CO3+0.5Fe2O3+(NH4)2HPO4+0.5C→NaFePO4+2.0NH3+0.5CO2+CO(2) Solid-Phase Synthesis MethodNaPO3+0.5Fe2O3+0.5C→NaFePO4+CO
Patent Document 2 indicates that, in both of the schemes (1) and (2), the carbon amount is about 100 wt % excess to the stoichiometric chemical reaction.    Patent Document 1: Japanese Unexamined Japanese Publication No. 2000-294238    Patent Document 2: Japanese National Patent Publication No. 2004-533706